Feedback amplifier circuits



CND 2,314,813

FEEDBACK AMPLIFIER CIRCUIT Filed July 30, 1938 JF-AKT 1Y0/foar ffsaaacx Snoentor dttorneg Patented Mar. 23, 1943 FEEDBACK AMPLIFEER CIRCUITS Donald S. Bond, Philadelphia, Pa., assgnor to Radio Corporation of America, a corporation of -Delaware Application July 30, 1938, Serial No. 222,092 2 claims.. (01.179-171') My invention relates to direct current amplifiers and more particularly to a direct current amplifier employing degenerative feedback.

I am aware that vacuum tube ampliiiers have been designed for the amplification of direct current voltages and currents. I vam aware also that various schemes have been suggested for maintaining stability notwithstanding variations in the supply voltages and differences in tube characteristics. Stability is a prime requisite in an amplifier which is to be used in making precision measurements. Stability must belhad in the presence of changes -which are caused by irregularities in the supply potentials.

One limitation on the accuracy of measuring equipment employing a direct current amplifier is the nonlinearity of the amplication charactei-istie with respect te the input voltage.- If the amplication vs. input voltage curve can be made linear over a wider range, the utility of the instrument is greatly increased. Not only does its operation on any particular range become better, but also there is a uniformity of calibration over a larger number of dierent ranges.

I have found that, by employing a.' degenerative feedback system of the type which i's described' hereinafter, it is possible to construct an amplifier which has a large degree of independence of gain with respect to variations in the supply voltage and differences in tube characteristics over a wide range of input voltages.

. It is an object of this invention to provide an improved direct current amplifier.

vIt is a further object of this invention to provide a; direct current amplier which is substantially independent of variations in supply potentials. t

It is 'another object of this invention to provide a combination of means for obtaining degenerative feedback. Y

It is a further object of this invention to provide a direct current amplifier whse outputyvolt-- age is a straight line function of the input vo1t.

age.

This invention will be better understood 'from the following description when considered in connection with the accompanying drawing. scope is indicated by the appended claims.

Referring to the drawing:A

Figure 1.is a schematic drawing of one embodiment of my invention, and

Figure 2 is a graph showing the input voltage versus plate'voltage characteristic of the device illustrated in Fig. l with andwithout feedback.

Referring-to Figure 1: A pair of input termi- 55 Its . the anode suppl;1 battery 11.

nals I, 3'a re connected, respectively, to the grids 5 and 'I of a pair of amplier triodes 9 and Il. A

pair of resistors I3 and I5 are connected in series between the grids 5 and 'I. The midpoint of the resistors I3 and I5 is connected to the negative terminal of an anode supply battery I'I, and also to both cathodes I9 and 2l through a bias resistor 23. A pair of feedback resistors 25 and 21 arerespectively connected between the grids 5 and 'I and the anodes 29 and 3l of the vacuum tubes. A pair of output terminals 33 and 35 are respectively connected to the two anodes 29 and 3|. A pair of plate load resistors 31 and 39 are connected in series between the anodes 29 and 3|. The midpoint of the two last-named resistors is connected to the positive terminalof It is evident from a study of the circuit diagram shown in Fig. 1 that-the resistors 25 and I3 constitute a voltage divider circuit to lmpress a positive voltage on the grid 5. The value of this voltage is, of course, dependent upon the plate potential and the relativel values of the two resistors. Resistors'21 and I5 likewise place a positive bias on grid 1. Consequently, apotential is impressed on the two grids 5 and 'I which than is customary in order to produce a voltage which is suflicient to overcomethe positive potential introduced by the voltage dividers mentioned above. The value of this resistor, for exfample, may be of the order of 50,000 ohms.

In theabsence' of an applied voltage the two tubes 9 and II draw equal anode currents and consequently have identical plate voltages. II a. voltage is applied to theV input terminals, one of 'the grids'will become positive with respect to the other. Since the cathodes of the two tubes are connected together, an unbalanced conditlon results causing one tube to draw more plate current while the other tube reacts in the opposite direction to draw less plate current. As a' result, a potential difference is established between the output terminals 33 and 35.

When grid 5 becomes more positive, the plate current of tube 9.increa's'es and thereby reduces its applied anode voltage. As a consequence,

the .potential applied to the gridthrough the4 'voltage dividing resistors 25 and I3 decreases from its former value.. The effect of this voltage is seen to be in a direction which tends to oppose the change due to the applied voltage. The feedback voltage is therefore said to be degenerative.

By a proper choice of resistors, the amount of feedback voltage can bemade suiiicient to reduce to any desired value variations in gain due to changes in the amplification factor of the Vacuum tubes or due to variations in line voltage land other causes. For example, resistors 25 and 21 are of the order of 1.5 megohms. Resistors I3 and I5 are of the order of 1.2 megohms. As a result, the overall amplification is maintained unione stage, it is evident that further similar stages may be added. I also prefer the push-pull arrangement illustrated, but it is evident that the feedback method illustrated is applicable to a form in the presence of varying factors which The curve is linear only over a limited range.

In contrast, a. second curve is shown which was obtained from the device shown in Fig. 1 and isA entitled with feedback. The improvement is evidenced by the fact that the operation curve is linear over a range of output voltage which is nearly twice as great as that of the amplifier without feedback.

I have found experimentally that when feedback is employed the total cathode current remains essentially constantv as the input voltage is varied. This is due to the fact that the increase in cathode current of one tube is substantially offset by a like decrease in the other tube. As a result, the negative grid bias remains constant. On the other hand, when feedback is not used, the cathode current rises rapidly as one tube reaches its cut-oil.' value.

In making the comparison shown in Fig. 2, the grid bias in each case was adjusted to the point that gave the greatest degree of linearity ofthe input vs. output curve.I Other bias -ad' justments could be made which givea higher gain over a limited range. For the optimum linearity, with and without feedback, the amplifier with feedback actually has higher gain than the amplifier without feedback. This is `shown by the greater slope of the with feedback curve of Fig. 2.'

While I have shown an.-ampliiier having but I single tube direct current amplifier. It is to be noted that in the push-pull amplifier shown, the input and output connectionsA are electrically symmetrical. 'I'hus the normal tendency of a single 'stage ampliner to reverse the sense of the input voltage can be corrected by merely reversing the output connections. This is possible because of the electrical symmetry of the circuit.

I claim as my invention:

1. A direct current amplifier comprising two space discharge tubes each provided with a cathode, a, control grid, and an anode, a common power supply for said tubes, a common cathode resistol` connected between both of said cathodes and one terminal of said power supply, a pair of grid resistors respectively connected ,between each of said control igrids and said one power supply terminal, means for applying an input current symmetrically to said pair of resistors so as to vary the voltages of said control grids in opposite directions with respect to said one power supply terminal in response to variations in' said input current, and two feedback resistors, each connected between the anode and the grid of one of said tubes, cooperating with the respective grid resistors to provide a feedback path for each of said tubes whereby a portion of the output voltage of each of said tubes'is applied to the control grid thereof to oppose variations in the voltage of said control grid.

2. A direct current amplifier comprising two space discharge tubes each provided with a cathode, a control grid, and an anode, a common power supply for said tubes, a common cathode resisto;` connected between both ofsaid cathodes and one terminal of said power supply, a pair of grid resistors respectively connected between each of said control grids and said one power supply terminal, means for applying an inputl current symmetrically to said pair of grid resistors so as to vary the voltages of s aid control ,grids in opposite directions with respect to said power supply terminal in response to variations in said input current. and a pair of feedback ,resistors respectively 'connected between the anode and the control grid of each of said tubes, cooperating with the respective-grid resistors to provide a voltage divider between the anode and the cathode of each of said tubes whereby a portion of the output voltage of each of said tubes' is applied to the control grid thereof. v

` DONALD S. BOND. 

